Second Generation Multiplexing

I want to devote a few lines to the second generation of truck multiplexing systems which were introduced by OEMs a little earlier this year. The Freightliner M2 chassis is an example of this technology but all the major truck OEMs are about to, or have, introduced second generation multiplexing. Multiplexing has been around in trucks in some form or other since the introduction of the first electronically conrolled engines in the late 1980s. The coach/bus industry got into second generation multiplexing ahead of trucks and continue to lead the way at this moment in time.

We'll begin with a short definition of multiplexing. In a multiplex electrical system,wire and hard wire switching components (sensors and switches) are replaced with electronic devices that communicate over a vehicle data bus. Rather than have individual wires transmitting voltage from switches to relays that then energize components, the multiplex system sends messages over the vehicle data bus. These messages are then interpreted by modules which can respond or not if the data is of relevance.

In the Freightliner M2 chassis, the J1939 data bus is the backbone or primary data bus of the system. It consists of a twisted pair of yellow and green wires with a terminating resistor at either end. There cannot be any open tee connector cavities: if J1939 communications are lost, the dash instrument cluster will display "no 1939".  An example of how a data bus transaction takes place is as follows.The engine ECM receives a signal from the TPS and broadcasts it on the data bus.  Because this data is of significance to the transmission, ABS, ICU, and CWS controllers, they can use it in developing their output logic. In the same way, the engine ECM receives information from the coolant temperature sensor and broadcasts it on the data bus. However, coolant temperature data is of no use to the CWS controller so it is simply not responded to. It is of significance to the ICU which responds by displaying its value on the instrument cluster.

Get the idea? Multiplexing is the key to replacing miles of hard wire with a simple pair of twisted wires called a data bus. Expect to see this technology develop rapidly and be prepared to adopt an entirely new mindset when troubleshooting electrical systems!